The fabrication of integrated circuits and other electronic components utilizes crystalline silicon, more specifically, single crystalline silicon, that possesses very high crystalline perfection. To control the quality of the single crystalline silicon, it is important to be able to determine the concentration of contaminates in polycrystalline silicon compositions from which the single crystalline silicon is formed.
One of the contaminates that affects the quality of the single crystalline silicon and that is typically analyzed through standard methods is carbon. A standard method of analyzing carbon concentration in crystalline silicon is set forth in ASTM F-1723-02. The crystalline silicon may be either polycrystalline silicon, single crystalline silicon, or a cross section of the single crystalline silicon, with different algorithms used to determine carbon concentration in the polycrystalline silicon and single crystalline silicon based on the carbon concentration in the cross section of the single crystalline silicon. A specific method of using infrared absorption to analyze carbon concentration in crystalline silicon is set forth in ASTM F 1391-93.
For the method of analyzing the carbon concentration in crystalline silicon, as set forth in ASTM F 1723-02, a polycrystalline silicon composition is provided. A polycrystalline silicon core, or ingot, is extracted from the polycrystalline silicon composition. The polycrystalline silicon core is then annealed at a temperature of 1360° C. for 2 hours. A 2 mm thick cross section is cut from the annealed polycrystalline silicon core. Carbon concentration in the 2 mm thick cross section is determined with a Fourier Transform infrared (FT-IR) spectrometer in accordance with ASTM F 1391-93. Based on the carbon concentration in the cross section, a bulk carbon concentration in the polycrystalline silicon composition may be determined based on well known algorithms.
Although current methods of analyzing carbon concentration in crystalline silicon provide useful results, the results are only sensitive to differences in carbon concentration of about 50 parts per billion atomic or greater. Certain cryogenic FT-IR analyses may be able to determine carbon concentration with greater sensitivity; however, the equipment required to perform cryogenic FT-IR analyses is expensive and requires precise environmental control to attain accurate results.
Due to the need for extremely pure crystalline silicon, and the need to determine the exact carbon concentration in crystalline silicon being used, it would be desirable to develop a new method of analyzing carbon concentration in the crystalline silicon with even greater sensitivity than 50 parts per billion atomic.